SCIEN C 



, 



in uliidi ;:n erect objict placed ncnr a hole in a card 

 in -xt i In- yc appears t<i It- <;-i t aide, and 



hl.-o inverted .-mil magnified. L-t <'!) I-'/, pi 

 cord perforated with a Mii.-.ll hole, K a wlrte v 

 window, 1) (lie rye of tl.> r. ami d thr h<ad of 



a pin lit-.i! near tin: eye and also near 1l:e hole in the 

 I't.di-r thc-e eimim-t m< es the pin d will be 

 it F inverted and minified. The reason of thif 

 is, as M. I A- Cat l\;is otiMTved, thnt the eye in this case 

 sees only the shadow of the pin on the retina, and 

 since the light which is stopped hy the tipper part of 

 the pin on its head comes from the lower part of the 

 white wall or window V, and that which is stopped by 

 the lower end of the pin comes from the upper part of 

 the wall or window K, the shadow must necessari-'y ap- 

 pear inverted with respect to the object.* 



The following variation of Le Cat's experiment has 

 been described hy Dr. Hrewster. Take a common pin 

 and hold it in any position near the eye so that the ob- 

 server sees reflected from its head a faint circle of light, 

 then hold a second pin opposite to it exactly as in l''i_ f . 

 19, and an inverted image of the one pin will be seen 

 in the head of the other. If the head of the first pm 

 is round and well polished, the inverted and magnified 

 image of the other will be more distinct. In this form 

 of the experiment a diverging pencil of li^ht from the 

 window or a candle replaces the diverging pencil in 

 Fig. ip. which proceeds from the perforation in the 

 card CB, and of course produces the same effect. The 

 little round knob, by the pressure upon which the case 

 of a watch is often opened, will answer better than the 

 finest pin head. Edin. Jour, of Science, No. VII. p. 89. 



15. On the Insensibility of the Retina to objects seen in- 

 directly, and to object it J'ttint /// it uminatcd. 



If we look, says Dr. Brewster.t at a narrow slip of 

 white paper placed upon a black or a coloured ground, 

 it will never appear to vanish, however lorg and at- 

 tentively we view it. But if the eye is fixed steadily 

 upon any object within two or three inches of the pa- 

 per, so as to see it only indirectly, or by oblique visiyn, 

 the slip of paper will occasionally disappear, as if it 

 had been removed entirely from the ground, the colour 

 of the ground extending itself over the part of the reti- 

 na occupied by the image of the slip of paper. 



If the object seen indirectly is a black stripe on a 

 white ground, it vanishes in a similar manner ; and, 

 what is still more remarkable, the same phenomena of 

 disappearance take place whether the object is viewed 

 with one or tvith both eyes. 



When the indirect object is luminous, like a candle, 

 it never vanishes entirely, unless it is placed at a great 

 distance ; but it swells and contracts, and is surround- 

 ed by a halo of nebulous light, so that the excitement 

 must extend itself to contiguous portions of the retina 

 which are not influenced by the light itself. 



If we place two candles at the distance of about eight 

 or ten ieet from the eye, and about twelve inches from 

 . each other, and view the one directly and the other in- 

 directly, the indirect image will be encircled with a 

 bright ring of yellow light, and the bright light within 

 the ring will have a pale blue colour. If the candles 

 are viewed through a prism, the red and green lights 



of the indirect image vanUh, and leave only n 

 mat* of yellow, terminated with a portion of blue !f!t 



While prrfortnitg thi* experiment, and 

 steadily awl directly M ? prismatic imapi-f 



the candle, I was surprised to observe that tlie rrd ami 

 green rays begin to disappear, leaving only yr///tc and 

 a small portion of Line ; and when the eye was kept 

 imn.oveab'y fixed on the same part of the image, the 

 yellow light became almost pure white, to that the 

 prismatic image wa* converted into an elongated image 

 cf white light. 



If the slip of white paper, viewed indirectly with 

 both eye, is placed so near as to be seen double, the 

 rays which proceed from it no longer fall on corres- 

 ponding points of the retina. In this case, the two 

 image* do not vanish simultaneously ; but when the 

 one begins to disappear, the other begins soon after it, 

 so that they sometimes appear to be extinguished at 

 the same time. 



In order to ascertain whether or not the accidental 

 colour of an object seen indirectly would remain after 

 t!.e object itself had disappeared, I placed a rectangular 

 piece of a red wafer upon a white ground, and having 

 looked steadily at an object in its vicinity, the wafer 

 disappeared, and though the accidental colour showed 

 itself just before the wafer had vanished, yet no trace 

 of colour was visible afterwards. 



The insensibility of the retina to indirect impretnott 

 has a singular counterpart in its insensibility to the <.'/- 

 reel impretsions of attenuated light. When the eye is 

 steadily directed to objects illuminated by a feeble 

 gleam of light, it is thrown into a condition nearly as 

 painful as that which arises from an excess of splen- 

 dour. A sort of remiss-ion takes place in the ccnvey- 

 ance of the impressions along the nervous membrane ; 

 the object actually disappears, and the eye is agitated 

 by the recurrence of excitements which are too feeble 

 for the performance of its functions. If the eye hail, 

 under such a twilight, been making unavailing efforts to 

 read, or to examine a minute object, the pain which it 

 suffers would admit of an easy explanation ; but, in the 

 present cA-e, it is the passive recipient of attenuated 

 hgbt, and the uneasiness which it experiences can arise 

 only from the recurring failures in the retina to trans- 

 mit its impressions to the optic nerve. 



The preceding facts respecting the affections of the 

 retina, while they throw considerable light on the 

 functions of that membrane, may serve to explain some 

 of those phenomena of the evanescence and reappear- 

 ance of objects, and of the change of shape of inani- 

 mate objects, which have beea ascribed by the vulgar 

 to supernatural causes, and by philosophers to the ac- 

 tivity of the imagination. If in a dark night, for ex- 

 ample, we unexpectedly obtain a glimpse of any ob- 

 ject, either in motion or at rest, we are naturally anxi- 

 ous to ascertain what it is, and our curiosity calls forth 

 all our powers of vision. This anxiety, however, serves 

 only to baffle us in all our attempts. Excited only by 

 a feeble illumination, the retina is not capable of afford- 

 ing a permanent vision of the object, and while we are 

 straining our eyes to discover its nature, the object will 

 entirely disrppear, and will afterwards appear and dis- 

 appear alternately. The same phenomenon may be 



See Le Cat's TVaiM det Sent, p. 298, and Priestley On J'WIOH, Vol. ii. p. 7iS. 

 f Edinburgh Journal of Science, Vol. iii. p. 288. 



* An analogous phenomenon, but arising from a quite different cause, must hare often been observed by persons who are very long-sight- 

 ed. In a dark night, the pupil dilates to such a degree M to depute the eye of its power of adjusting itself to moderate distances, ts-ce 



VOL. XVII. PART II. *4 r 



